Build the Blocks
Dry-lay a ring of blocks on the fire pit site, placing them end to end until you have a perfect circle positioned where you want the finished pit to be. To adjust the size of the circle, you may need to cut a block. Hold the block over the gap it will fill, then mark it on the underside at the proper width.
Using a 3-inch cold chisel and a brick hammer, score the block on the mark, and continue the score all the way around the block. Place the block on a hard surface (flat rocks or gravel). Hold the chisel in the score line, then hit it with the brick hammer until the block splits.
Clean up jagged edges with the tail of the brick hammer. Place the cut block into the ring.
Mark the Pit Location
Make sure all the joints between the blocks are tight and the front and back edges line up. Using a spade, mark a circle in the ground about an inch outside the perimeter of the ring.
Take note of how many stones make up the ring, then remove them and set them aside.
If the blocks you are using are interlocking, remove any tongues on the bottom of the first-course blocks so they will lie flat in the trench. Chip them off with the tail of a brick hammer.
Create a Level Trench for the Blocks
Using a spade, dig a straight-sided trench, 12 inches deep and as wide as one block, within the circle marked out on the ground. Then dig down 6 inches in the area encircled by the trench.
Lay the ring of blocks in the trench to see if all the pieces fit in a circle. If not, dig more to widen the trench. Remove blocks.
Fill the Trench
Fill the trench with 6 inches of 3/4-inch drainage gravel. Using a hand tamper, compact the gravel. If necessary, add more gravel to keep the trench level and even.
Tip: Always make sure the blocks line up perfectly in the front and back when you lay them out; a difference of 1 inch in the circle’s diameter could create a 3-inch gap between blocks.
Lay and Level the First Course
Place the first block in the ring. Using a 2-foot level, check that it sits level both side to side and front to back. Where the block is too high, tap it down with a rubber mallet. Where it’s too low, shim it slightly with a handful of patio base. Make sure this first block is perfectly level and positioned correctly in the trench before moving on.
Lay another block next to the first one. Butt the sides together tightly and line up the front and back edges. Using the first block as a reference, level the second block side to side and front to back.
Lay the rest of the blocks in the trench in this manner until the ring is complete and all the blocks you counted earlier are used. Make sure each block is perfectly leveled and lined up tight with its neighbor before moving on to the next one. (You may have to coax the last block into place with a mallet.) Using a 4-foot level, occasionally check level across the ring.
A small hit with a mallet can make a big adjustment; work slowly and carefully, block by block.
Assemble the Walls
Using a caulking gun, squeeze a zigzag bead of masonry adhesive across two adjacent blocks. Lay a block on top of the glue-covered pieces, centering it over the seam between the two. Make sure any interlocking parts on the blocks fit together well. Continue until the second course is finished.
Fill the Pit
Fill the pit with 6 inches of gravel, which will help support the first two courses as they set up. Glue and lay the third and fourth courses, continuing to stagger the joints.
Insert the iron campfire ring into the circle. Adjust it to sit even with the top of the block wall. Fill any space between the ring and the block wall to the top with gravel.
Work quickly and only in a small area at one time; masonry adhesive sets quickly.
Cap the Blocks
Loosely arrange the cap pieces on top of the pit walls. (If you are using natural stone, try to arrange the pieces together like a puzzle.) Lay one stone edge over the next and mark the upper stone where they meet. Also, roughly mark the stone for a 2-inch overhang on the outside of the circle and an inch on the inside. Using a brick hammer and a chisel, score the stone on those marks. On thick natural stone, use a grinder fitted with a diamond blade to score it more deeply.
Lay the stone on a hard surface. Split it by hitting a chisel in the score mark, or by tapping against the stone’s edge with the brick hammer until it breaks. Score and split each stone this way, moving around the circle in one direction until you’ve made a cap that fits together tightly.
If you’re using blocks, glue the pieces on top of the wall. If you’re using natural stone, combine the dry mortar with enough bonding additive—not water—to make a mix with a peanut-butter consistency.
Wet the wall with some bonding agent. Lay a large mound of mortar on two blocks. With the point of the trowel, make a groove across the mortar. Lay the capstone on top, push it down, then tap it with the rubber mallet to set and level it. Continue to lay the capstones in this manner until the wall is finished. Wait two days before lighting a fire.